Recently, imaging apparatuses using low coherence interferometry have been put in practical use. This is called an OCT (Optical Coherence Tomography or Optical Coherence Tomographic method).
In an opthalmologic field, the OCT is used in order to obtain a tomography image of a fundus of the eye or its vicinity. In addition to the opthalmologic field, the OCT is used in an attempt to observe a tomography image of a skin, or to capture a wall surface tomography image of a digestive organ, a circulatory organ, and the like by incorporating the OCT in an endoscope or a catheter.
As a type of the OCT, a method called TD-OCT (Time Domain OCT: a TIME DOMAIN method) is disclosed in U.S. Pat. No. 5,321,501. This method will be described briefly with reference to FIG. 32.
Here, FIG. 32 is a schematic diagram illustrating a TD-OCT.
Light radiated by a radiation unit 3201 is divided into reference light and signal light by a division unit 3202. The reference light is reflected by a movable reference mirror 3203. As illustrated in the figure, the movable reference mirror 3203 is moved in a one-dimensional direction mechanically to define a measured position inside an inspection object 3205 in an optical axis direction of signal light incident on the inspection object 3205.
Through a light beam scanning optical system 3204, the signal light impinges on and is reflected by the inspection object 3205. The light beam scanning optical system 3204 uses the signal light incident on the inspection objects 3205 to scan in a predetermined direction. Reflected light from the movable reference mirror 3203 and that from the inspection object 3205 interfere with each other, and the interference light is detected by a detection unit 3207 to determine information with respect to the inspection object 3205.
TD-OCT is a method of constructing image data based on strength data of the successively obtained interference light by performing an A-scan (axis-directional scanning of incident light into an inspection object, or depth-directional scanning in the inspection object) by the movable reference mirror 3203.
One-dimensional data can be obtained continuously through the A-scan by causing the light beam scanning optical system 3204 to scan the inspection object 3205 with the signal light incident on the inspection object in one direction (for example, x-direction) in a plane of the object.
Then, a two-dimensional tomography image can be obtained using images obtained continuously. In addition, a three-dimensional image can be obtained by using the above-mentioned signal light to scan in two directions (e.g., an x-direction and a y-direction) within the above-mentioned plane.
Here, although it is necessary to move the movable reference mirror 3203 at high speed in order to increase measuring speed by the TD-OCT, there is a mechanical limit in acceleration of the movable reference mirror 3203.
In addition, as another type of OCT, a method called an SD-OCT (Spectral Domain OCT: Spectral domain method) is disclosed in “Handbook of Optical Coherence Tomography” (2006) (FIGS. 2 and 3 in pages 145 and 149, and FIG. 1 in page 338). This method will be described briefly with reference to FIG. 33.
Here, FIG. 33 is a schematic diagram illustrating the SD-OCT. In FIG. 33, constructions different from FIG. 32 are a point that a movable reference mirror is a fixed reference mirror 3308, a point of using a spectroscopes 3309, such as a diffraction grating, and a point that a detection unit is a spectral detection unit 3310, such as a line sensor. Reference numeral 3305 denotes an inspection object, reference numeral 3306 does a measurement region, reference numeral 3304 does a scanning optical system, reference numeral 3302 does a light dividing unit, reference numeral 3308 does a reflection unit, and reference numeral 3301 does a light source.
SD-OCT is a method of acquiring image data temporally in a lump by detecting a spectrum, dispersed by the spectroscope 3309, by the spectral detection unit 3310, and performing Fourier transformation of the coherent light intensity information with respect to a wavelength axis into information with respect to a tomographic position axis. Since this SD-OCT can obtain image data in a depth direction inside the inspection object 3305 in a lump, it is possible to increase measuring speed in comparison with the TD-OCT which performs serial scanning temporally in the depth direction.
Here, the SD-OCT (Spectral Domain method) is one kind of an FD-OCT (Fourier domain optical coherence tomography), and, besides this, there is an SS-OCT (Source Swept-OCT).